In recent years, imaging optical devices represented by digital still cameras and digital video cameras (simply referred to as “digital cameras”, hereinafter) have spread rapidly. For example, a large number of such digital cameras are commercialized that have the number of recorded-image pixels greater than 5 million. Among these, digital cameras of reduced size and weight are desired in particular.
According to demand for such further size and weight reduction of the imaging optical devices, recently, the following lens systems are proposed (Japanese Laid-Open Patent Publication No. 2004-212616, Japanese Laid-Open Patent Publication No. 2004-252204 and Japanese Laid-Open Patent Publication No. 2005-24844).
Each of the lens systems described in Japanese Laid-Open Patent Publication No. 2004-212616, Japanese Laid-Open Patent Publication No. 2004-252204 and Japanese Laid-Open Patent Publication No. 2005-24844 is a zoom lens system, in order from the object side to the image side, comprises: a first lens unit having positive optical power; a second lens unit having negative optical power; a third lens unit having positive optical power; and a fourth lens unit having positive optical power. Then, the second lens unit is, in order from the object side to the image side, constructed from three lens elements of a first lens element having negative optical power, a second lens element having negative optical power and a third lens element having positive optical power. As such, in each of the lens systems described in Japanese Laid-Open Patent Publication No. 2004-212616, Japanese Laid-Open Patent Publication No. 2004-252204 and Japanese Laid-Open Patent Publication No. 2005-24844, the second lens unit is constructed from minimum lens elements, and size and weight are reduced.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-212616
Patent Document 2: Japanese Laid-Open Patent Publication No. 2004-252204
Patent Document 3: Japanese Laid-Open Patent Publication No. 2005-24844